The aim is to optimize and validate the Magnetic Resonance Imaging (MRI) bolus passage perfusion technique in stroke victims. Recently discovered perfusion/diffusion MRI techniques are among the most promising developments in acute stroke care in this decade because they can potentially distinguish the ischemic core from surrounding hypoperfused but salvageable tissue. At present, the weakest component of the acute stroke MRI study is the perfusion study. There has been no direct demonstration that perfusion MRI accurately detects penumbral hypoperfusion by comparison with a "gold standard" method in stroke patients. No team has defined a recipe of optimal perfusion scanning parameters and then validated this recipe in stroke victims against a gold standard. There has been no direct validation of the usually-made assumption of a log-linear relationship between intravascular contrast agent concentration and MRI signal change. Likewise, no proven data analysis recipe has come forth. Oxygen-15 water Positron Emission Tomography (PET) blood flow assessment will be used as gold standard to validate perfusion MRI studies of stroke victims. MRI and PET assessment of blood flow will be made back-to-back. Medically stable stroke patients in the subacute epoch (< 48 hrs) will be studied. These patients are expected to display a wide range of blood flow values which will provide a high dynamic range over which to test the accuracy of perfusion MRI, yet be stable enough to undergo both studies. MRI perfusion scans using different MRI scanning parameters and analysis methods will be evaluated against the PET gold standard. Using the validated MRI procedures, a further pilot study is planned to explore whether MRI can accurately identify perfusion abnormalities and cerebrovascular reserve deficiencies in asymptotic subjects who have vascular risk factors for stroke. UCLA has new state-of-the-art MRI and PET imaging devices together with active collaborating teams having expertise in stroke clinical management (both acute and rehabilitative), in MRI scanning and in PET scanning.